1. Field of the Invention
This invention relates to alloys, and a method for producing alloys for use in nuclear reactors and more specifically this invention relates to alloys and a method for producing alloys having low ductile to brittle transition temperatures (DBTT).
2. Background of the Invention
Nuclear reactor environments are among the harshest on materials and substrates contained therein. There, temperatures of more than 250° C. occur. These environments also experience pressures of more than 2 psi. Irradiation fluence exposure values of more than 2×1021 n/cm2 (E>0.1 MeV) are common.
As a consequence of these harsh conditions, substrates consisting of pure elements fall short of providing robust structural forms with long lifetimes. For example, while such elements as chromium, iron, niobium, tungsten, and molybdenum have extremely low coefficients of thermal expansion and high thermal conductivity, their resistance to other aspects of a nuclear core (irradiation exposure, pressure) make them suboptimal choices. Neutron irradiation embrittlement limits the service life of materials comprising some reactor-pressure vessels in commercial nuclear-power plants.
Irradiation embrittlement results from the nucleation and growth of defect clusters, as these clusters restrict the movement of metal atom dislocations along the lattice which are needed for ductile deformation. As such, the flow stress is elevated above the inherent fracture stress of the material and brittle fracture is observed at temperatures where ductile deformation is normally seen. Defect clusters are formed by the aggregation of point defects (vacancies and self-interstitial atoms) created by the displacement of atoms from their lattice sites by collisions with high-energy neutrons during irradiation in an operating nuclear reactor. Indeed, pre-radiation exposure DBTTs (between about −100° C. and −50° C.) of some molybdenum alloys increase to more than 800° C. DBTT after exposure to irradiation fluence exposure levels typically found in reactor cores.
Reactor core environments include neutron fluence exposure values of between 2×1021 n/cm2 (E>0.1 MeV) and 1×1023 n/cm2 (E>0.1 MeV) for more than one month, and at temperatures exceeding 250° C. Therefore, elements, alloys and other substrates for use within the nuclear reactor environment must withstand high temperature, high pressure and high irradiation exposure. However, the resistance to these harsh conditions is often short lived. For example, while commercially available unalloyed molybdenum and commercially available TZM Mo-alloy exhibit DBTTs of between −50 and −100° C., irradiation results in a constant upward shift in the DBTT. Alloys such as Mo—Cr exhibit DBTTs of more than 800° C. after irradiation.
Chromium was thought to be a desirable dopant to molybdenum substrates, inasmuch as it is greatly undersized with respect to molybdenum (1.18 A for Cr versus 1.30 A for Mo), and inasmuch as the mobility of chromium is comparable to or faster than the point defects produced by irradiation. Stress fields created by Cr solute atoms serve as pinning sites. These sites pin or slow down point defects that block the movement of dislocations. The inability of dislocations to glide through the microstructure of the alloy causes increased brittleness.
However, the ductility of Mo—Cr alloys has been reported to be poor when the chromium content is greater than 0.1 percent. Specifically, Mo—Cr alloys with chromium contents greater than 0.1 percent have been reported to have a DBTT of between about −129° C. and room temperature, which is too high to be useful in advanced reactor designs.
A need exists in the art for an alloy with pre-irradiation ductile-to-brittle transition temperatures no higher than −50° C. The alloy should be comprised of high levels of chromium (i.e., greater than 0.1 weight percent) but without the heretofore concomitant embrittlement associated therewith after irradiation.